Devices of the above kind are known, for example, from Swedish Patent Specification No. 356,407 and British Patent Specification No. 1,444,774. However, in these known devices a disturbance (e.g. an interference pulse) on the input signals may generate a pulse on the output of the device, resulting in unintentional tripping. Furthermore, determination of the resistance and the reactance in separate circuits on separate measuring occasions requires the use of a large number of components in the device.
If phase comparators of the integrating type are used for reducing the effect of the said disturbances, these integrations will affect the shape of the output signal requiring the use of time-determining components of high accuracy. In addition, the integrations considerably increase the operating times in the case of distorted input signals.
The present invention aims to provide an impedance-measuring device of the kind referred to which does not have the above-mentioned drawbacks.